I am really hooked on watching Seinfeld; it is really my favorite series. Now in one of the episodes the sentence in bold is used:

George: Hello.
Kelly the Waitress: Well, what's it going to be?
George: What's it gonna be?
Kelly the Waitress: Yes. What'll you have? Are you eating? It's in that vein.
George: I’ll eh, I’ll just have a bowl of chili.

I always wondered what that means. I couldn't even understand it, I had to search for the script of the episode, and look at what was actually said.

The original phrase is It's in a similar vein, which actually started in mines since ores naturally formed long streaming deposits called veins. Miners would use the phrase to effectively communicate the locations of each separate vein. So if a miner uncovered what looked like two different deposits, a senior miner may come down and tell him that one deposit is "in a similar vein" to the other.

Incidentally, Barrie England's explanation and my own are in a similar vein; The deposits were so named for their striking resemblance to blood vessels.

However, the origin is just fun trivia in case you wanted to explain it to someone else, as it's not a phrase I hear often anymore. At least you now understand that the phrase denotes a relationship or connection between things. In Kelly's case, her waitress vernacular for May I take your order

She's trying to explain that, when she says "What's it going to be?," she means it as a synonym for "What will you have?" or "Are you eating?" When two concepts are closely related, they are in the same vein.